The present invention relates to a sheet metal interstage casing for a pump a sheet metal, and more particularly to an interstage casing for a pump which is pressed into shape for use in a multistage centrifugal pump.
Conventionally, there is known an interstage casing for a pump in which the casing is formed of sheet metal such as a stainless steel plate and manufactured by press work.
This type of interstage casing is shown in FIG. 2 of the accompanying drawings. As shown in FIG. 2, the interstage casing is of a cylindrical receptacle-like structure comprising a cylindrical side wall 1 and a bottom wall (or casing end wall) 2 on an end thereof (on the right-hand side in FIG. 2) which is connected to a subsequent (or the next) interstage casing. The axial ends of the cylindrical receptacle-like structure are machined into a bottom end surface 3 joined to the bottom wall 2 and an open end surface 4. The bottom wall 2 has a radially outer cylindrical surface 5 to be fitted in the next interstage casing, providing a spigot joint. The open end surface 4 has a radially inner cylindrical surface 6 fitted over the radially outer cylindrical surface 5 of a preceding interstage casing, providing a spigot joint. These surfaces 5, 6 are also machined to desired dimensional accuracy.
The interstage casing houses a impeller 7 having an inlet end disposed in the opening of the cylindrical side wall 1 which is defined by the open end surface 4. That is, the inlet side of the impeller 7 is disposed in confronting relation to the bottom wall 2 of the preceding interstage casing. The interstage casing also accommodates a return blade 8 with a side plate 9 joined thereto, the return blade 8 being welded at plural spots 10 to the surface of the bottom wall 2 which faces the impeller 7.
The impeller 7 can be rotated by a shaft 11. A liner ring 12 is attached to the bottom wall 2. A shaft sleeve 14 is fitted over the shaft 11. The side plate 9 is mounted on the shaft sleeve 14 through a bearing or bushing 13.
When the multistage centrifugal pump is in operation, the liquid to be pumped is pressurized by the impeller 7, passes through a passage defined in the return blade 8 between the side plate 9 and the bottom wall 2, and is led to the next impeller by which the liquid is further pressurized. The pressure of the liquid is applied to the reverse side of the bottom wall 2 as indicated by the arrows P, tending to deform the bottom wall 2 radially inwardly toward the lower-pressure side (toward the left-hand side in FIG. 2).
If the bottom wall 2 is deformed to a large extent, then welded spots 10 between the return blade 8 and the bottom wall 2 may be subjected to excessive stresses that may rip off the welded spots 10. To prevent the bottom wall 2 from being deformed excessively, the interstage casing has a stiffener plate 2A welded to the bottom wall 2. The return blade 8, the side plate 9, and the bottom wall 2 are also increased in thickness to prevent them from being deformed excessively.
FIG. 3 of the accompanying drawings shows in cross section an interstage casing including a spherical bottom wall 2 which has a relatively thin wall thickness. Those parts shown in FIG. 3 which are identical or similar to those shown in FIG. 2 are denoted by identical or similar reference characters.
FIG. 4 of the accompanying drawings shows in fragmentary cross section a vertical-shaft multistage centrifugal pump comprising interstage casings each of the structure shown in FIG. 2. The interstage casings are assembled together by a fastening band 15. The multistage centrifugal pump includes a discharge port 16 and a cable 17.
When the multistage centrifugal pump is in operation, the liquid to be pumped is drawn from a suction port (not shown) and pressurized by the successive impellers 7. The pressure head of the liquid is restored as the liquid passes through each of the return blades 8. Finally, the liquid is discharged out of the pump through the discharge port 16.
Of various forces induced by the liquid pressure and pressure differences applied to the interstage casings, the most problematic would be the force imposed by the interstage pressure difference acting on a flat portion normal to the shaft between adjacent ones of the interstage casings. Heretofore, such force has not caused any substantial problem because the interstage casings have been formed by casting.
As described with reference to FIG. 2, however, the interstage casing pressed from sheet metal suffers various drawbacks when the bottom wall 2, which corresponds to the flat portion referred to above, is deformed. More specifically, the pressure P that has been increased by the impeller 7 is applied to the bottom wall 2, thus deforming the bottom wall 2 in the direction of the force p radially toward the lower-pressure side. When the bottom wall 2 is thus deformed, very large stresses are developed in the welded spots 10 between the bottom wall 2 and the return blade 8.
Consequently, the pressure that can be increased by a single impeller is determined by the extent to which the flat portion (bottom wall) is deformed. Therefore, the interstage casing cannot be greatly increased in size, and should require a considerable wall thickness.
On the other hand, the spherical bottom wall that has a spherical shape to thereby reduce deformation and is employed to make the thickness relatively thin, as shown in FIG. 3, is also disadvantageous in that the return passage for the liquid is not of a good shape, resulting in a reduction in the performance of the pump.